Autoimmune diseases are approaching epidemic levels, estimated to affect 5-8% of the U.S. population. Pathogenesis is attributed, in large part, to self-reactive T cells that recognize auto-antigens in affected tissues and secrete destructive, pro-inflammatory cytokines. Consequently, the differentiation of naive T cells into pro- inflammatory versus tolerogenic T helper cell lineages regulates the immunologic state of the host. T cell receptor (TCR) signals, along with local cytokines, are required for initiating T helpr cell differentiation, and altering the TCR signal strength influences effector lineage commitment. Understanding the precise molecular mechanisms contributing to TCR signal strength and T helper cell differentiation is important when considering immunotherapies for T cell-mediated diseases, particularly autoimmunity. We recently demonstrated that Map3k8 transduces TCR signals in naive T cells and helps to specify a Th1 transcriptional program. What is not clear is precisely how Map3k8 impinges upon the multiple TCR signaling pathways to regulate the development and functions of other T helper cell lineages. The goal of this proposal is to determine how the serine-threonine kinase Map3k8 influences TCR signaling, T helper cell differentiation, and contributes to autoimmunity. Our central hypothesis is that Map3k8 acts as a rheostat to modulate the strength of TCR signals and thereby alters lineage commitment and effector functions of T cells in vivo. Aim1 will examine how Map3k8 influences T helper cell differentiation using in vitro T cell polarization assays. Aim 1 seeks to determine which TCR signaling pathways are defective in Map3k8-/- mice and how they contribute to T helper cell differentiation using gene expression assays, Western blotting, transcription factor translocation and rescue of the phenotype with lentiviral expression of specific signaling molecules. Aim2 will determine how Map3k8 contributes to T cell-mediated autoimmunity using both animal models and pharmacologic inhibitors and will address whether Map3k8 functions similarly in T helper differentiation of human cells. Knowledge gained about the role of Map3k8 in TCR signaling networks will not only contribute to our fundamental understanding of normal T cell development and functions, but will also provide insight into the pathogenesis of autoimmune diseases that may ultimately elicit innovative approaches to their treatment and prevention.